In general, a press is a device which applies a force to a workpiece, where the force may vary from position to position during one cycle of the press. By way of example, various press types include presses for forming, punching, mold closing, compressing, bending, drawing or injection molding. In general, to monitor and control presses of these types, two variables (dimensions) are monitored at the press. The first variable is an absolute position in the press cycle. Absolute position may be the position of the tool associated with the press. Typically, position in a press having a rotating drive mechanism whose angular position is directly related to the absolute position in the press cycle is determined by a position feedback device or transducer such as an encoder or resolver which provides a signal representative of a position in the press cycle. For presses lacking a rotating drive mechanism whose angular position is directly related to the absolute position in the press cycle, a linear resolver is typically used to provide a signal which is representative of the linear position of a press. The second variable is force. Force may be monitored by a sensor or transducer exposed to fluid pressure or to strain in the press or its tooling in such a way as to produce a signal related to the process force necessary to effect the process performed by the press.
Apparatus and methods for monitoring and controlling presses are known. In particular, U.S. Pat. No. 4,987,528, issued to Michael J. O'Brien on Jan. 22, 1991 (hereinafter referred to as the '528 patent) discloses a signature analysis control system for a press. This system monitors forces at the press throughout the press cycle and may initiate an action such as shutting off the press if the monitored forces are determined to be unacceptable. To determine whether or not forces at a press are acceptable for a given cycle of the press, the force signature for a given cycle of the press is compared to arrays of upper and lower reference values. The upper reference values may be developed by adding a set of offset values to a reference signature, and the lower reference values may be developed by subtracting a second set of offset values from the reference signature. Other types of press monitoring and/or control systems are discussed in U.S. Pat. No. 3,257,652 issued to George B. Foster on Jun. 21, 1966; U.S. Pat. No. 3,680,365 issued to McGarvey G. Summers on Aug. 1, 1972; U.S. Pat. No. 4,016,744 issued to Williams et al. on Apr. 12, 1977; U.S. Pat. No. 4,023,044 issued to Miller et al. on May 10, 1977; U.S. Pat. No. 4,048,848 issued to Frank R. Dybel on Sep. 20, 1977; U.S. Pat. No. 4,059,991 issued to Dybel et al. on Nov. 29, 1977; U.S. Pat. No. 4,062,055 issued to Dybel et al. on Dec. 6, 1977; U.S. Pat. No. 4,088,899 issued to Miller et al. on May 9, 1978; U.S. Pat. No. 4,116,050 issued to Tanahashi et al. on Sep. 26, 1978; U.S. Pat. No. 4,171,646 issued to Dybel et al. on Oct. 23, 1979; U.S. Pat. No. 4,177,517 issued to Mette et al. on Dec. 4, 1979; U.S. Pat. No. 4,195,563 issued to Budraitis et al. on Apr. 1, 1980; U.S. Pat. No. 4,207,567 issued to Richard O. Juengel on Jun. 10, 1980; U.S. Pat. No. 4,260,986 issued to Kobayaski et al. on Apr. 7, 1981; U.S. Pat. No. 4,289,022 issued to Dybel et al. on Sep. 15, 1981; U.S. Pat. No. 4,445,093 issued to Robert D. Kohler on Apr. 24, 1984; U.S. Pat. No. 4,453,421 issued to Yasuhiro Umano on Jun. 12, 1984; U.S. Pat. No. 4,504,920 issued to John Mickowski on Mar. 12, 1985; U.S. Pat. No. 4,519,040 issued to Brankamp et al. on May 21, 1985; U.S. Pat. No. 4,524,582 issued to Lucas et al. on Jun. 25, 1985; U.S. Pat. No. 4,527,156 issued to Nawrocki et al. on Jul. 2, 1985; U.S. Pat. No. 4,554,534 issued to William J. Jones on Nov. 19, 1985; U.S. Pat. No. 4,633,720, issued to Dybel, et al. on Jan. 6, 1987; U.S. Pat. No. 4,695,965 issued to Fujita et al. on Sep. 22, 1987; U.S. Pat. No. 4,721,028 issued to Lucas et al. on Jan. 26, 1988; U.S. Pat. No. 4,723,429 issued to Weber et al. on Feb. 9, 1988; U.S. Pat. No. 4,750,131 issued to Miguel R. Martinez on Jun. 7, 1988; U.S. Pat. No. 4,766,758 issued to Lucas et al. on Aug. 30, 1988; U.S. Pat. No. 4,939,665 issued to Gold et al. on Jul. 3, 1990; and German Patent Document No. DE 3715077 A1.
While press controllers having various schemes for monitoring stamping presses are known, it is important to provide a press control which monitors the sequence of loads which occur at the press and analyze these loads in such a way which does not cause spurious shutdowns of the press. Typically, press controls are used in high volume press operations to ensure the quality of the products produced by the press. Additionally, these presses may operate in the range of 500-1,000 cycles per minute, where shutting down the press, due to inaccurate monitoring of the press, results in substantial losses of money due to production shutdown.
However, while the method for analyzing the forces at a press must be tolerant enough to avoid spurious and unnecessary shutdowns, this tolerance must not be so large as to render the press controller ineffective with respect to detecting faulty parts at the press, or problems within the operating mechanisms of the press itself. Accordingly, while a number of systems are available for monitoring and analyzing the forces at a press, it would be desirable to provide a system for monitoring the forces at a press which avoids analyzing press forces in such a way to produce undesirable press shutdowns while also accurately detecting faulty parts produced by the press and problems in the operating mechanisms of the press.
As discussed above, presses which are typically fitted with press controls are high production presses. Thus, while it is desirable to sample the force at a press throughout a press cycle, and as frequently as possible, this task is difficult due to the limitations imposed by press control hardware, e.g., the speed of the hardware under many circumstances is not sufficient to monitor and analyze the forces in a high speed press with enough resolution with respect to force and position in the cycle to be effective. Accordingly, it would be advantageous to provide a press control which is capable of variably or selectively changing the position resolution for portions of the press cycle which are particularly important, e.g., increasing the position resolution during a critical portion of a press cycle such as the portion of the cycle where a tool is operating on a workpiece, as opposed to the portion of the cycle where the press is merely bringing the tool toward the workpiece. By increasing the position resolution during the portion of the press cycle where the tooling is operating on the workpieces, the profile or signature of the forces used in the formation of the finished workpiece is more accurately recorded.
Another problem which is encountered with press controls for press operations is the situation where a press, such as a hydraulic press or press driven by a crankshaft, undergoes a reversal in the position of the tool due to elasticity in the press and mechanism for driving the tool. By way of example, this situation may occur during punching operations when the tool breaks through the material being punched.